1. Field of the Invention
The present invention relates generally to techniques for recovering the heavy metal constituents from the waste stream of certain metal finishing operations, and more particularly, to a novel means of precipitating such metals as hydrous oxides.
2. Description of the Prior Art
The ideal water pollution control technique for metal finishing operations, such as electroplating, aluminum anodizing or conversion coating, would be one which is simultaneously a recovery technique. Most often, pollution control techniques (those which render the effluent in compliance with discharge regulations) are quite different from recovery techniques and the two terms have, in practice, come to be understood as mutually exclusionary. Present state-of-the-art compliance technology results in a grossly contaminated metal-containing product which is unsuitable for simple recovery of the heavy metal constituents. This is due to the incorporation of additional chemical agents for purposes of rendering the effluent in compliance with applicable discharge regulations without regard to the recoverability of the heavy metal constituents. However, recent changes in EPA solid waste disposal regulations have compelled the user to implement recovery procedures. Recovery techniques require supplementary technology to assure that the by-product of recovery is in compliance with applicable discharge regulations. However, even the best of recovery techniques does not yield a recovery product easily suitable for reintroduction into the process bath.
Evaporation of process rinsewaters was implemented by industry as a solution to the combined compliance and recovery problem. It was thought that by simply removing the water, the contaminated waste could then be reintroduced into the process bath. Many factors, however, proved to work against this technique. Energy requirements proved too high, air pollution problems abounded, degradation of sensitive bath components developed, and the impurities resident in both bath and raw rinsewater concentrated themselves, often presenting a poisoning threat to the bath.
The predominant recovery techniques of reverse osmosis and ion exchange fall far short of being sufficiently discriminatory to exclude the resident impurities in the process bath. Reintroduction of the permeate or column eluate concentrates the impurities anew. Eventually a point of intolerance is reached whereby the process must be shut down and purification undertaken.
Precipitation as a hydrous oxide is highly advantageous from a cost perspective but it does not provide the proper means of generating a crystalline solid. A severe limitation to the precipitation of principal heavy metals as hydrous oxides is that the zone of metastable precipitation is virtually nonexistent. Precipitation cannot therefore be effected from homogeneous solution and as a result, hydrous oxide precipitates are difficult to filter and highly impure. One of the most common yet ineffective means of generating pure and dense hydrous oxides is to add strong caustic soda (sodium hydroxide) solution to a solution of the heavy metal. The requirements of volume minimization dictate that a concentrated caustic soda feed be employed. Typically, 50% solutions of this substance are employed for this purpose. Yet addition of such a concentrated sodium hydroxide solution generates hydrous oxide nuclei so rapidly that the precipitates have highly disadvantageous qualities. Customary attempts to render these precipitates well behaved by the use of flocculants only serves to contaminate the product and make the hydrous oxide unsuitable for facile recovery. The use of commercial grade caustic soda also introduces impurities into the system which accumulate in the harvested hydrous oxide.
The ideal technique for pollution control would be a technique which would recover all bath components in a facile manner. Such a technique is unachievable due to the differing nature of the components of the production bath, requiring the employment of highly distinct recovery processes. However, a compliance technique which would enable simultaneous facile recovery of the principal heavy metal bath components, if commercially available, would be of significant value. An object of the present invention is to provide such a technique. A further object of the present invention is to provide a technique for precipitating such heavy metals in an easily handled and filterable form without the introduction of chemical agents which can introduce impurities into the harvested metals. Another object of the present invention is to provide a technique which promotes the formation of crystals in regular form and at a rate which excludes occluded impurities.